To estimate a maximum purchase price for the coin in question, we will consider several factors:

1. Starting Price: 90 euros.
2. Condition: The coin has a condition rating of 20 (BC-), indicating that it is not in very good shape.
3. Auction History:
- It has been auctioned 4 times and has sold on all occasions.
- The average hammer price in previous auctions is 73.75 euros.
- Previous hammer prices have been 90 euros (2020), 20 euros (2017), and 85 euros (2016).

Given that the starting price is 90 euros and considering that the average hammer price in the past is 73.75 euros, it seems that the starting price is quite high compared to the historical hammer price data.

However, the highest recorded hammer price is 90 euros, suggesting that there is interest in this coin at that price. Since the condition is low, it is likely that collector interest will be limited.

Taking all these factors into account, an estimated maximum purchase price could be around 80-85 euros, as this reflects interest in the coin while also considering its condition and the historical hammer prices.